Results from a new study performed in mice indicate that hybrid laboratory
antibodies derived from chimpanzees and humans may provide a potentially safe
and effective way to treat the serious complications that can occur following
smallpox vaccination — and possibly may even protect against the deadly disease
itself. The study, led by researchers with the National Institute of Allergy
and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH),
appears online this week in the Proceedings of the National Academy of Sciences
(PNAS).

A worldwide immunization program officially eradicated naturally occurring smallpox
disease in 1980. However, concerns of a bioterror attack involving the highly
contagious and fatal virus have prompted researchers to search for new smallpox
vaccines and treatments.

The currently licensed smallpox vaccine consists of a live but weakened strain
of vaccinia virus, a relative of the variola virus that causes smallpox. Vaccinia
immunization has been proven effective in generating immunity against smallpox
virus and other orthopoxviruses, including monkeypox and cowpox.

Although most reactions to the vaccinia virus are mild, the vaccine can cause
serious and even life-threatening complications in individuals with weakened
immune systems or skin conditions such as eczema, in infants younger than 12
months and in pregnant women. Health care providers currently treat smallpox
vaccine complications with anti-vaccinia immune globulin (VIG) — pooled antibodies
taken from the blood of individuals immunized with the smallpox vaccine. However,
VIG is in short supply since the United States discontinued its public smallpox
vaccination program in 1972.

NIAID-funded researchers have been working to develop alternatives to VIG based
on antibodies they created in the laboratory. The study appearing online this
week in PNAS details how senior authors Robert H. Purcell, M.D., co-chief of
NIAID’s Laboratory of Infectious Diseases, and Bernard Moss, M.D., chief of NIAID’s
Laboratory of Viral Diseases, and their collaborators developed hybrid antibodies
from chimpanzees and humans that effectively inhibited the spread of both vaccinia
and variola viruses in test tube experiments. Moreover, the hybrid antibodies
proved more effective than VIG when tested in mice infected with vaccinia virus,
even when given two days after virus exposure.

“This is an important finding in the race to develop effective measures against
a potential bioterror attack involving the deadly smallpox virus,” says NIH Director
Elias A. Zerhouni, M.D.

“It is imperative that we have effective treatments available that everyone
could use in the event of a bioterror attack,” says NIAID Director Anthony S.
Fauci, M.D. “This study shows that there are potential alternatives to existing
treatments and perhaps to existing vaccines that we can use to enhance our arsenal
of medical countermeasures.”

Using a library of antibodies derived from the bone marrow of two vaccinia-immunized
chimpanzees, the study researchers identified a pair of potent antibodies that
target and neutralize the B5 protein, one of five key proteins responsible for
cell-to-cell spread of infectious vaccinia virus. The researchers then combined
the two chimp-derived antibodies with a human antibody to create two hybrid test
antibodies, 8AH7AL and 8AH8AL. In test tube experiments, both antibody types
prevented the spread of vaccinia virus. Further, the 8AH8AL antibody neutralized
one strain of the smallpox-causing variola virus. The test involving the smallpox
virus was performed at the Centers for Disease Control and Prevention in Atlanta.

The researchers then tested the effectiveness of the hybrid antibodies in mice.
The control group — mice that were given the vaccinia virus but did not receive
the antibodies — experienced continuous weight loss for five days after virus injection,
which the researchers correlated with viral replication in the lungs. In contrast,
mice injected with either of the two types of hybrid antibodies did not lose
weight.

Since there was no difference in the protective abilities between the two hybrid
antibodies, the researchers used 8AH8AL to determine the minimum effective dose.
Groups of mice were given decreasing doses — 90, 45 and 22.5 micrograms (micrograms)
per mouse — of 8AH8AL or a single 5-mg dose of human VIG (two and a half times
the recommended human dose on a weight basis) as a point of comparison. All five
mice in the control group died or were sacrificed when their weight fell to 70
percent of their starting weight. All of the mice that were injected with 8AH8AL
(even at the lowest dose) or with VIG were protected from death.

Further, mice that received a single 90-microgram dose of 8AH8AL two days after
virus exposure experienced only slight weight loss followed by rapid recovery.
Conversely, all five of the mice that received 5 mg of VIG 48 hours after virus
exposure experienced much greater weight loss than those that received the hybrid
antibody.

“This study demonstrated that the hybrid antibodies provide instant protection
against the vaccinia virus and likely smallpox and are potentially more potent
and more specific than the treatment we currently have available,” says Dr. Purcell.
The hybrid antibodies also offer a potentially significant advantage over VIG
as a treatment for smallpox vaccination complications not only because VIG is
in limited supply but because VIG lots may have different potencies and carry
the potential to transmit other infectious agents, he adds.

According to Dr. Purcell, the hybrid antibodies should be tested in another
animal model for effectiveness against the monkeypox virus, which closely mirrors
smallpox but is less virulent in humans.

Currently, the smallpox virus exists in only two laboratories found in Atlanta,
Georgia, and in Russia.

News releases, fact sheets and other NIAID-related materials are available on
the NIAID Web site at http://www.niaid.nih.gov.

NIAID is a component of the National Institutes of Health, an agency of
the U.S. Department of Health and Human Services. NIAID supports basic and
applied research to prevent, diagnose and treat infectious diseases such as
HIV/AIDS and other sexually transmitted infections, influenza, tuberculosis,
malaria and illness from potential agents of bioterrorism. NIAID also supports
research on transplantation and immune-related illnesses, including autoimmune
disorders, asthma and allergies.

The National Institutes of Health (NIH) — The Nation's Medical Research
Agency — includes 27 Institutes and Centers and is a component of
the U. S. Department of Health and Human Services. It is the primary Federal
agency for conducting and supporting basic, clinical, and translational medical
research, and it investigates the causes, treatments, and cures for both common
and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov.